1. Field of the Invention
This invention relates to ohmic contact electrodes for semiconductor diamonds such as diodes, transistors and sensors.
2. Description of the Prior Art
The semiconductor diamond is now drawing an industrial attention as a new material for semiconductor devices such as diodes, transistors, sensors, or the like.
Although the diamond is widely known as an insulating substance, the one discussed herein is the semiconductor diamond having a low resistivity.
The diamond has a wide forbidden band gap (5.5 eV) as well as a large carrier mobility (2000 cm.sup.2 /Vs), and is stable thermally and chemically.
The diamond is normally an insulator, but may be made as the semiconductor of relatively low resistance by doping impurities.
For these reasons, the semiconductor diamond is keenly expected as a material for environmentally withstanding, high-frequency, high-power and or blue light emitting devices.
The semiconductor diamond is available in the forms of a natural bulk, a high-pressure synthetic bulk, and a vapor-phase synthetic film.
The p-type diamond can be obtained by doping boron (B).
On the other hand, the n-type diamond, though it may be obtained by doping phosphorus (P) and lithium (Li), has a high resistivity. And an n-type diamond with a low resistivity has not yet been developed.
Although bipolar devices are not involved, the devices that have already come out in prototypes on the basis of the semiconductor diamond include at present Schottky diodes, making use of Schottky junction with tungsten (W) and a p-type diamond, and several types of unipolar transistors.
In manufacturing semiconductor devices, it is extremely important to form such electrodes that allow ohmic contact. The contact electrodes of anodes and cathodes of light emitting devices or source and drain electrodes of field effect transistors must be ohmic contact electrodes. The ohmic contact electrode means an electrode the current-voltage characteristics through which is symmetric between the forward and backward directions in accordance with the Ohm's law. Moreover, it is strongly desired that the resistance thereof be low. It is common knowledge that an electrode capable of ohmic contact can be obtained by forming some metals such as Ti, AuTi, and AuTa through the method of metallizing on a p-type diamond.
The technique of forming ohmic contacts is an important technique in manufacturing semiconductor devices.
However, the ohmic contact now available to semiconductor diamonds using Ti, AuTa, and the like involve a too large contact resistance as much as 1 .OMEGA.cm.sup.2 or more.
In any device manufactured using the conventional ohmic contact with the semiconductor diamond, even if a voltage is applied to, there occurs a remarkable voltage drop across such an ohmic contact region. This results in a unnegligible voltage drop of an effective voltage applied to the device, whereby good characteristics of the semiconductor diamond contact can not be sufficiently utilized. Furthermore, the device suffers from a serious drawback that heat is generated at the ohmic contact region.
In manufacturing any electronic devices, contact resistances less than 10.sup.-4 .OMEGA.cm.sup.2 are desirable for that of ohmic contacts, and far lower contact resistances are demanded for high-speed, high-frequency devices.
To enable the semiconductor diamond to be utilized as a material for semiconductor devices, a low-resistance ohmic contact is essential.